Myofibroblasts are specialized contractile cells that participate in tissue fibrosis and remodeling, including idiopathic pulmonary fibrosis (IPF). Mechanotransduction, a process by which mechanical stimuli are converted into biochemical signals, regulates myofibroblast differentiation. Relaxin is a peptide hormone that mediates antifibrotic effects through regulation of collagen synthesis and turnover. In this study, we demonstrate enhanced myofibroblast contraction in bleomycin-induced lung fibrosis in mice and in fibroblastic foci of human subjects with IPF, using phosphorylation of the regulatory myosin light chain (MLC 20) as a biomarker of in vivo cellular contractility. Compared with wild-type mice, relaxin knockout mice express higher lung levels of phospho-MLC 20 and develop more severe bleomycin-induced lung fibrosis. Exogenous relaxin inhibits MLC 20 phosphorylation and bleomycin-induced lung fibrosis in both relaxin knockout and wild-type mice. Ex vivo studies of IPF lung myofibroblasts demonstrate decreases in MLC 20 phosphorylation and reduced contractility in response to relaxin. Characterization of the signaling pathway reveals that relaxin regulates MLC 20 dephosphorylation and lung myofibroblast contraction by inactivating RhoA/Rho-associated protein kinase through a nitric oxide/cGMP/protein kinase Gdependent mechanism. These studies identify a novel antifibrotic role of relaxin involving the inhibition of the contractile phenotype of lung myofibroblasts and suggest that targeting myofibroblast contractility with relaxin-like peptides may be of therapeutic benefit in the treatment of fibrotic lung disease. © 2011 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.